1. Field of the Invention
The present invention relates to sensors and devices for determining the flow characteristics of a multi-phase fluid flowing along a pipe, and more particularly to such devices for measuring the dielectric constant of the fluid, the volume fraction or xe2x80x9choldupxe2x80x9d of each of the components of the fluid, and the speed and the flow rate of each of the various phases.
2. Description of the Related Art
When monitoring wells for extracting hydrocarbons, it is important to be aware of the quantity of water produced simultaneously with the hydrocarbons.
To evaluate the water and hydrocarbon flow rates in homogeneous flows in pipes, three quantities must be estimated, namely, the mean water volume fraction HW, the mean water speed vW, and the mean hydrocarbon speed vo. The flow rates are then as follows:
qw=AHWvW
for the water; and
qo=A(1xe2x88x92HW)vo
for the hydrocarbon, where A is the section of the pipe.
When the flow is not homogeneous, which is possible in deviated wells, flow-rate evaluations based on the above equations are invalid. It is then necessary to take account of the effective distribution of the velocities and of the volume fractions across the section of the well. Such an approach means that a plurality of devices are placed across a given cross-section of the pipe.
It is also known that the speed of a flow in a pipe can be determined by measuring a magnitude that varies over time s1(t) and s2(t) at two different cross-sections of the pipe, and then by calculating a cross-correlation function:
C= less than s1(t)*s2(t) greater than 
In a two-phase fluid, the fluctuations in the magnitude s(t) may, for example, be due to inhomogeneous structures propagating along the pipe at the mean speed of the flow.
If T is the value of t at the maximum of the cross-correlation function C, the speed v of the flow is given by:
v=L/T
where L is the axial distance between the two measurement sections.
It is also known, e.g. from U.S. Pat. No. 5,017,879, that capacitive devices can be used to determine the characteristics of multi-phase flows. The dielectric constant of a mixture of fluids depends on the respective fraction of each of its components and on their individual dielectric constants. It has thus been proposed to estimate the composition of a two-phase fluid on the basis of its dielectric constant.
The dielectric constant is itself obtained by exciting the fluid by means of electrodes separated by the fluid, in particular electrodes placed on the pipe, and across which an AC voltage is applied. The measured magnitude is the resulting current.
Guard electrodes have also been added to maintain the electrostatic field between the active electrodes. It is thus easier to interpret the measurements by limiting the edge effects due to the finite length of the active electrodes, or by focussing the electric field in a particular zone of the flow.
In both of the above-mentioned cases, namely when the flow is not homogeneous, or when the speed is measured, it is thus necessary to dispose a plurality of devices, in particular capacitive devices, close together on the pipe. Contradictory requirements then have to be faced.
It is desirable to use devices that are of small size. In a non-homogeneous flow, better resolution in space is thus obtained, thereby considerably improving the speed and the accuracy of the reconstruction algorithm. When speed is measured, the small size of the devices makes it possible to bring them closer together, and thus to obtain a correlation peak that is clearer for the resulting measurements, because the inhomogeneous structures deform to a lesser extent between the two devices.
Unfortunately, such a small size generally makes the measurements much more sensitive to electromagnetic noise. When the measurements are capacitive measurements, the measured capacitance values are low. Typically, the currents induced by the stray capacitance may be greater by several orders of magnitude than the current resulting from the capacitance to be measured. The stray capacitance thus gives rise to a systematic error or bias whose variations can exceed the amplitude of the signal itself.
An object of the present invention is to mitigate those drawbacks.
More particularly, an object of the invention is to provide capacitive sensors for determining the characteristics of multi-phase flows, which sensors are of small size while being substantially insensitive to noise and thus substantially free from systematic error, as well as measurement devices including such sensors.
To this end, the invention firstly provides a capacitive sensor for determining the flow characteristics of a multi-phase fluid in a pipe, said sensor comprising at least one excitation electrode provided with at least one cutout in which at least one measurement electrode is disposed, said electrodes being organized to be applied against said pipe.
The invention also provides a device for capacitively measuring the dielectric constant of a multi-phase fluid flowing along a pipe, said device comprising at least one sensor as described above, means for maintaining said electrodes at the same potential and for measuring the current output by said measurement electrode, and means for deducing said dielectric constant from said current.
This configuration then makes it easy for all of the conductors liable to give rise to interference that are situated in the vicinity of the device in the detection system to be maintained at the potential of the excitation electrode. These conductors are thus at the same potential as the measurement electrode. The load thereon thus depends only on the potentials applied to the active electrodes.
Two embodiments are considered.
In a first embodiment, the excitation electrode is connected to the general ground of said power supply means. This solution is advantageously simple.
In the other embodiment, the excitation electrode constitutes the floating ground for said means for measuring the current. The advantage of this embodiment is that the signal can be brought by amplification to a level at which it dominates the common mode rejection voltage of the amplifier.
In this other embodiment, the means for measuring the current may comprise a first amplification stage referenced relative to the potential of the excitation electrode, and a second amplification stage organized to bring the reference of the output signal to the general ground.
More particularly, the device may include shielding electrically connected to the excitation electrode, around the measurement electrode and around said first amplification stage.
It should be observed that such a configuration does not require the first amplification stage to be located in the immediate vicinity of the measurement electrode. The shielding of said measurement electrode may be extended by a shielded cable along which a conductor passes that connects the measurement electrode to the amplification means which are themselves provided with shielding forming the following portion of the shielded cable.
Said excitation electrode may overlap the measurement electrode.
The invention further provides a device for capacitively measuring the volume fraction of a first component of a two-phase fluid flowing along a pipe, said device comprising at least one device as described above for measuring the dielectric constant of said fluid, and means for deducing said volume fraction from said dielectric constant.
It is known that, in a homogeneous mixture, and provided that the drops of a conductive fluid are immersed in an insulating fluid, the value of the dielectric constant em is related to the value of the dielectric constant eo of the continuous phase and to the volume fraction 1xe2x88x92HW of said continuous phase by the following relationship:       e    m    =            e      0        ⁢          xe2x80x83        ⁢          1                        (                      1            -                          H              w                                )                3            
In a mixture of water and of hydrocarbon in a well, this relationship applies if the well is substantially vertical.
More particularly, such a volume fraction-measuring device may comprise a plurality of devices as described above for measuring the dielectric constant of said fluid, said devices in said plurality of devices being distributed over the periphery of a cross-section of said pipe.
It is then possible to obtain a section across the pipe. In a hydrocarbon well, the above relationship is no longer applicable but it is no longer necessary for the well to be vertical.
The invention also provides a capacitive device for measuring the speed of a two-phase fluid flowing along a pipe, said device comprising at least two devices as described above for measuring the dielectric constant of said fluid, the devices being disposed in different cross-sections of said pipe, and means for establishing a cross-correlation between the measurements delivered by said two devices and for deducing said speed therefrom.
More particularly, said at least two devices for measuring the dielectric constant of said fluid are disposed substantially along the same generator line of said pipe, and they have a common excitation electrode.
Since the signal does no have any systematic error, the measurement electrodes may be small. Typically, the electrodes may have dimensions of about one centimeter. Furthermore, the common excitation electrode guarantees that the monitored zone is homogeneous. Finally, no minimum gap is necessary between the various measurement electrodes associated with the same excitation electrode.
The invention further provides a device for capacitively measuring the flow rate of a two-phase fluid flowing along a pipe, said device comprising at least one device as described above for measuring the volume fraction of a first component of said fluid, at least one device as described above for measuring the speed of said first component, and means for deducing said flow rate therefrom.
Naturally, the device for measuring the volume fraction, and the device for measuring the speed may use the same electrodes.
The invention also provides a method of measuring the dielectric constant of a two-phase fluid flowing along a pipe, said method comprising the steps of placing two electrodes on either side of a portion of said fluid, and applying an AC voltage across the two electrodes, wherein at least one excitation electrode is provided with at least one cutout in which at least one measurement electrode is disposed, and further comprising the steps of maintaining said excitation electrode and said measurement electrode at the same potential, measuring the current flowing between said electrodes, and deducing said dielectric constant from said current.
Particular embodiments of the invention are described below by way of non-limiting example and with reference to the accompanying diagrammatic drawings, in which: